cow milk or heavy whipping cream
To make sour cream, take one cup of fresh cream (or, if you don't have a milk cow, buy heavy whipping cream from the store). Mix it with one tablespoon of sour-cream (this is where the culture comes from).
You need to keep this mixture, in whatever container you mixed it in, at about 70 degrees for 24 hours. If the top of your fridge is warm you could set it there.... I just fill up a bowl with hot water and put the container in there (usually have to weight it down or it will float). After 24 hours it will have thickened considerably. (If not, the temperature was too low... just get it to 70 degrees and let it sit until it thickens). Then put it in the fridge. It is best to refridgerate it for 24 hours before using it.
Once you have your sour cream, you can use a tablespoon of it to get your next batch going...